Various mRNAs have been shown to localize in specific cytoplasmic regions (Singer, Curr. Biol. 3:919-721 (1993); Wilhelm et al., J. Cell Biol. 123:269-274 (1993)). Known examples of such mRNA localization fall into two major classes: cellular mRNAs transcribed in somatic (or zygotic cells); and maternal mRNAs that are asymmetrically positioned in oocytes, where they are involved in establishing embryonic axes.
Maternal mRNA localization is exemplified by oskar and bicoid mRNAs, which localize, respectively, at the posterior and anterior ends of Drosophila eggs (Macdonald, Sem. Dev. Biol. 3:413-424 (1992)). In Drosophila, mutations affecting the localization of the oskar and bicoid mRNAs are lethal.
Actin provides a useful model of mRNA localization in somatic cells. Actin is a highly abundant structural constituent of all eukaryotic cells. It is integral to a variety of cellular functions. In differentiating mammalian myoblasts, .beta.-actin mRNA localizes to the leading lamellae, while .alpha.-actin localizes to a perinuclear compartment of the same cytoplasm (Kislauskis et al., J. Cell Biol. 123:165-172 (1993)).
As a major constituent of the cytoskeleton or myofilaments, actin is essential for the maintenance of cell polarity and motility (Bretcher, Ann. Rev. Cell Res. 7:337-374 (1991); Cooper, Ann. Rev. Physiol. 53:585-605 (1991); Levitt et al., Mol. Cell Biol. 7:2457-2466 (1987); Pollard et al., Curr. Opin. Cell Biol. 5:1-2 (1993)), intracellular transport (Kuznetsov et al., Nature 356:722-725 (1992)), protein synthesis (Hesketh et al., Biochem. J. 277:1-10 (19910; Negrutskii et al., Proc. Natl. Acad. Sci. USA 88:4991-4995 (1991); Yang et al., Nature 347:494-496 (1990)), enzymatic processes (Farwell et al., J. Biol. Chem. 265:18546-18553 (1990); Hunt et al., Biochim. Biophys. Acta 1043:19-26 (1990); Knull et al., Curr. Top. Cell Regul. 33:15-30 (1992); Sarndahl et al., J. Cell Biol. 109:2791-2799 (1989)), and mRNA localization (Singer, Curr. Opin. Cell Biol. 4:15-19 (1992); Sundell et al., J. Cell Biol. 111:2397-2403 (1991); Yisraeli et al., Development 108:289-298 (1990)) .
In chicken embryonic fibroblasts ("CEFs") and myoblasts, actin mRNA is highly localized at the leading lamellae, quite different from the distribution of mRNAs coding for either vimentin or tubulin (Lawrence et al., Cell 45:407-415 (1986)). In leading lamellae, rapid changes in actin polymerization drive extension of the lamellipodia (Carlier, J. Biol. Chem. 266:1-4 (1991); Cooper, supra; Wang, J. Cell Biol. 105:2811-2816 (1987)). Both .beta.-actin protein and mRNA colocalize at the leading edge of endothelial cells in response to wounding (Hoock et al., J. Cell Biol. 112:653-664 (1991)) and in C2 myoblasts (Hill et al., J. Cell Biol. 122:825-832 (1993)). Thus, actin mRNA localization may facilitate the compartmentalization of actin synthesis (Singer, supra).
Cytoplasmic localization of maternal mRNAs depends on their 3' untranslated regions ("3' UTRs") (Jackson, Cell 74:9-14 (1993)). For example, a 50 nucleotide sequence, designated BLE1, found in the 680 nucleotide bicoid 3' UTR, has been implicated in bicoid mRNA localization (Macdonald et al., Development 118:1233-1243 (1993)).
Although the 3' UTRs of .alpha.-actin and .beta.-actin mRNAs have been shown to be necessary and sufficient for proper cytoplasmic localization (Kislauskis et al., supra), those 3' UTRs are large, i.e., .beta.-actin, 591 nucleotides; .alpha.-actin, 175 nucleotides. Which part (or parts) of the 3' UTR is responsible for the localization of .beta.-actin mRNA (or any other cellular mRNA) to specific cytoplasmic regions has remained unknown.